Enhanced liver X receptor signalling reduces brain injury and promotes tissue regeneration following experimental intracerebral haemorrhage: roles of microglia/macrophages

Abstract

BackgroundInflammation-exacerbated secondary brain injury and limited tissue regeneration are barriers to favourable prognosis after intracerebral haemorrhage (ICH). As a regulator of inflammation and lipid metabolism, Liver X receptor (LXR) has the potential to alter microglia/macrophage (M/M) phenotype, and assist tissue repair by promoting cholesterol efflux and recycling from phagocytes. To support potential clinical translation, the benefits of enhanced LXR signalling are examined in experimental ICH.MethodsCollagenase-induced ICH mice were treated with the LXR agonist GW3965 or vehicle. Behavioural tests were conducted at multiple time points. Lesion and haematoma volume, and other brain parameters were assessed using multimodal MRI with T2-weighted, diffusion tensor imaging and dynamic contrast-enhanced MRI sequences. The fixed brain cryosections were stained and confocal microscopy was applied to detect LXR downstream genes, M/M phenotype, lipid/cholesterol-laden phagocytes, oligodendrocyte lineage cells and neural stem cells. Western blot and real-time qPCR were also used. CX3CR1CreER: Rosa26iDTRmice were employed for M/M-depletion experiments.ResultsGW3965 treatment reduced lesion volume and white matter injury, and promoted haematoma clearance. Treated mice upregulated LXR downstream genes including ABCA1 and Apolipoprotein E, and had reduced density of M/M that apparently shifted from proinflammatory interleukin-1β+to Arginase1+CD206+regulatory phenotype. Fewer cholesterol crystal or myelin debris-laden phagocytes were observed in GW3965 mice. LXR activation increased the number of Olig2+PDGFRα+precursors and Olig2+CC1+mature oligodendrocytes in perihaematomal regions, and elevated SOX2+or nestin+neural stem cells in lesion and subventricular zone. MRI results supported better lesion recovery by GW3965, and this was corroborated by return to pre-ICH values of functional rotarod activity. The therapeutic effects of GW3965 were abrogated by M/M depletion in CX3CR1CreER: Rosa26iDTRmice.ConclusionsLXR agonism using GW3965 reduced brain injury, promoted beneficial properties of M/M and facilitated tissue repair correspondent with enhanced cholesterol recycling.

LXR agonism for CNS diseases: promises and challenges

“…In our recent study, daily administration of 10 mg/kg GW3965 for 7 days after collagenase induced ICH inhibited the expression of IL-1β and iNOS at perihematomal region. More importantly, GW3965 promoted the phenotype shifting of perihematomal microglia/macrophages from proinflammatory (IL-1β + Iba1 + ) to regulatory phenotype (Arg1 + /CD206 + CD68 + ) at day 7 post-ICH; this phenotype change was associated with improved histological and functional recovery because conditional depletion of microglia/macrophages abrogated the therapeutic effects of LXR agonism [ 49 ].…”

“…In LPS-primed human and mouse peripheral blood mononuclear cells (PBMCs), intracellular cholesterol crystals increased the expression of IL-1β in a concentration-dependent manner [ 51 ]. Moreover, the downregulated proinflammatory genes and microgliosis in experimental intracerebral hemorrhage were associated with reduced accumulation of crystals and lipid droplets [ 49 ]. Even though the direct proof is lacking to conclude that excessive cholesterol from myelin degradation and cellular debris worsen neuroinflammation in different CNS pathologies, it is plausible to hypothesize that microglia and macrophages may act in this manner.…”

“…In a model of bilateral carotid artery stenosis, 10 mg/kg intraperitoneal injection of GW3965 promoted proliferation of neural stem cells in the dentate gyrus during chronic cerebral hypoperfusion, and improved performance in Morris water maze test [ 63 ]. In collagenase induced experimental ICH, 7-day treatment with 10 mg/kg GW3965 increased the density of SOX2 + Ki67 + neural stem cells and SOX2 + DCX + neuronal progenitor cells in subventricular zones [ 49 ]. In a triple-transgenic mouse model of Alzheimer’s disease, 50 mg/kg GW3965 affected gene methylation related to neurogenesis and synaptogenesis including that of Sox-3 and syn1, which may contribute to the improvement in cognitive function [ 64 ].…”

“…In organotypic cerebellar cultures demyelinated by lysolecithin, both T0901317 and 25-hydroxycholesterol induced activation of LXRs and increased the expression of mature oligodendrocyte markers including myelin basic protein, and the number of myelinated axons [ 72 ]. In experimental ICH, GW3965 treatment increased the density of OPCs and mature oligodendrocytes at perihematomal regions and improved white matter integrity [ 49 ], which was abolished by microglia/macrophage conditional depletion [ 49 ]. The underlying mechanisms may be related to alleviated inflammatory responses and enhanced reuse of cholesterol.…”

“…The depletion of CD206 + regulatory myeloid cells using mannosylated clodronate liposomes inhibited oligodendrocyte differentiation in vivo, and conditioned medium from regulatory (exposed to IL-10 or IL-13) microglia promoted OL differentiation and maturation in both cell and organotypic cultures [ 73 ]. In collagenase induced ICH, GW3965 reduced the expression of proinflammatory markers such as IL-1β and iNOS while promoted Arg1 + and CD206 + regulatory microglia/macrophages, which was linked to increased oligodendrocyte lineage cells [ 49 ]. Moreover, the unresolved neuroinflammation and cytokines including IL-1β may induce continuing cell death and other cascading responses, which could be challenging for the survival and maturation of OPCs, and remyelination [ 50 ].…”